Update #1
Alright, as promised here is where I'm at. The structure is still very rough but if you look at the middle tower in the illustration you can see the form taking shape. I did add a mini to give you an idea of scale.

I'm trying to keep the base as light as I can, hence using the wine boxes which is fairly strong cardboard. The R4 foam is eventually going to be covered with paper-mache and then gravel and small stones.

I was getting a bit frustrated with the project so I started at the top and decided to work down as you can see in the photographs near the bottom of this post. I did make a mistake with the upper floor which I added the wooden popsicle sticks. I'll need to correct that (I need to cut out stairs on the right hand side too).

In terms of overall height its 27". For reference my Nippon castle is 23" in height. Based on the illustration the middle tower is the highest point with the lower bailey gatehouse being roughly the same size. This (roughly) works out to being about 1/2 of what it would be to scale. I figure if it was truly at scale based on the illustration the cliffs alone would have to be 4 feet.

Since I've taken these photos I have completely shingled the roof which is removalable.

Good start! This will be massive, as is proper. Have you considered adding a weathercock on one of the towers? Perhaps a gibbet with a desiccated skeleton dangling from a beam protruding from a castle wall on the other side (i.e. the one not facing the river)? Maybe a bird's nest hidden somewhere as an easter egg?

If you somehow had the space to spare, then it would phenomenal if you could extend the base of the mount to include the river with trees and trolls and a guard in the foreground, so as to be able to recreate the artwork in three dimensions. But that would eat a lot of space! Fitting magnus opus project though, for the megalomaniac fantasy builder.

Good start! This will be massive, as is proper. Have you considered adding a weathercock on one of the towers?

Indeed.

Some sort of accoutrement is going to go on the peak of the roof.

Admiral Wrote:

Perhaps a gibbet with a desiccated skeleton dangling from a beam protruding from a castle wall on the other side (i.e. the one not facing the river)?

Have you been looking at my painting desk??? Exactly what I'm doing. Just so happens I have the gibbet from the giant sprue.

Admiral Wrote:

If you somehow had the space to spare, then it would phenomenal if you could extend the base of the mount to include the river with trees and trolls and a guard in the foreground, so as to be able to recreate the artwork in three dimensions. But that would eat a lot of space!

Not sure about that, but I am planning on including the dungeon and boat-dock areas